Asus ROG Phone II Replacement Battery C11P1901 3.85V 5900mAh

Asus ROG Phone II — 3.85V Li-Polymer Replacement Battery (C11P1901)

This is a 3.85V, 5900mAh (22.72Wh) Li-Polymer replacement battery for the Asus ROG Phone II and ROG Phone II Strix. It fits the original battery slot using the same connector and BMS interface as the factory cell. It replaces OEM part numbers C11P1901 and 0B200-03510300.

• ROG Phone II and Strix compatibility: Both the standard ROG Phone II and the Strix edition use the same battery bay geometry, connector pinout, and BMS handshake protocol — the cell is interchangeable across the full lineup without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the ROG Phone II platform. The BMS accepted the cell without error flags, and the charge IC engaged correctly at both standard and fast-charge current levels.
• First-cycle fast charge protocol: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell.

Sudden shutdown at 20–30% on the ROG Phone II after a cell swap

The ROG Phone II uses a coulomb counter in its fuel gauge IC that was calibrated to the original cell's internal resistance and discharge curve. A new cell has different impedance characteristics, so the IC underestimates remaining capacity. Under GPU or modem load, the cell voltage drops sharply — hitting the hardware cutoff threshold before the displayed percentage reaches zero. One full discharge-to-cutoff and charge cycle resets the coulomb counter against the actual cell curve. After that cycle, the shutdown threshold aligns correctly with the new cell.

USB-PD fast charge not activating on the first cycle after replacement

After installation, some units report that fast charging is not offered — the charger falls back to standard 5V/2A input. This happens because the charge IC queries the BMS on the new cell before it has completed an initial handshake cycle, and the BMS responds conservatively until it has one calibration pass. Charge the phone once at standard speed to let the BMS complete its first cycle. On the next charge session, fast charging should engage normally at the expected input wattage.